A 3rd generation partnership project long term evolution (3GPP LTE) (hereinafter, referred to as ‘LTE’) communication system which is an example of a mobile communication system to which the present invention can be applied will be described in brief.
FIG. 1 is a diagram illustrating a network structure of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) which is an example of a mobile communication system. The E-UTRAN system is an evolved version of the conventional UTRAN system, and its basic standardization is in progress under the 3rd Generation Partnership Project (3GPP). The E-UTRAN system may be referred to as a Long Term Evolution (LTE) system.
Referring to FIG. 1, the E-UTRAN system includes a eNBs (e-NodeB or base station), which are connected with each other through an interface X2. Also, each of the eNBs is connected with a user equipment (UE) through a radio interface and connected with an evolved packet core (EPC) through an interface S1.
The EPC includes a mobility management entity (MME), a serving-gateway (S-GW), and a packet data network-gateway (PDN-GW). The MME includes access information of the user equipment or ability information of the user equipment. The access information or the ability information is mainly used for mobility management of the user equipment. The S-GW is a gateway having the E-UTRAN as an end point, and the PDN-GW is a gateway having a packet data network (PDN) as an end point.
Layers of a radio interface protocol between the user equipment and a network may be classified into a first layer L1, a second layer L2 and a third layer L3 on the basis of three lower layers of OSI (open system interconnection) standard model widely known in communication systems. A physical layer belongs to the first layer L1 and provides an information transfer service using a physical channel. A radio resource control (RRC) layer belongs to the third layer and serves to control radio resources between the user equipment and the network. To this end, the RRC layer allows the user equipment and the base station to exchange RRC messages with each other.
Although the wireless communication technology developed based on WCDMA has been evolved into LTE, request and expectation of users and providers have continued to increase. Also, since another wireless access technology is being continuously developed, new evolution of the wireless communication technology is required for competitiveness in the future. In this respect, reduction of cost per bit, increase of available service, use of adaptable frequency band, simple structure, open type interface, proper power consumption of user equipment, etc. are required.